In 3GPP (Third Generation Partnership Program) reference signal design for LTE (Long Term Evolution) Rel-10 is currently under discussion. In order to support up to 8 layer transmissions as well as CoMP (Coordinated multi-point) an extended set of UE specific demodulation reference signals (DRS) will be defined. Furthermore, a new set of reference signals is being discussed for channel state information feedback (CSI-RS). The CSI-RS that are currently proposed in 3GPP are sparse in time and frequency and can be DTX:ed (DTX—Discontinuous Transmission) without breaking any critical system functions.
LTE Rel-8 supports both open-loop as well as closed-loop multi-stream transmission in the downlink, also referred to as open-loop spatial multiplexing and closed-loop spatial multiplexing. In case of closed-loop spatial multiplexing a user equipment (UE) reports a recommended number of layers, Rank Indication (RI), and a recommended pre-coder matrix, Pre-coder-Matrix Indication (PMI) to an eNodeB (eNB) to assist selection of a suitable pre-coder matrix for transmission. The eNB can follow the recommendations, but is not obliged to do so. If the recommendations are not followed the eNB explicitly informs the UE what pre-coder matrix it will use for transmission. In contrast, open-loop spatial multiplexing (also often referred to as large-delay CDD), neither requires detailed feedback from the UE regarding recommended pre-coding nor any explicit signaling from the eNB to the UE regarding pre-coder selection.
One issue with the prior art closed-loop transmission schemes is that when the bandwidth is increased (up to 100 MHz in Rel-10) at the same time as the number of layers (up to 8 in Rel-10) increase, the gain in relation to the overhead becomes questionable in some scenarios. First of all, in case of full rank transmission then it is of less importance which pre-coding matrix is chosen. In case of full rank transmission it is not possible to have an array gain on all layers and hence the performance of the different pre-coders that may be selected will become fairly similar. The pre-coding may help in orthogonalizing the channel, but this effect is usually quite limited unless the pre-coders can very accurately match the instantaneous channel realizations.
A second issue with the prior art closed-loop transmission schemes is related to the fact that when the bit-rate is increased then effectively the number of transmission bursts that may be considered too small for achieving a closed loop performance boost is increased. It takes some time before the UE has measured the CSI (Channel State Information) and reported it to the eNB so that the eNB can adjust the transmission pre-coders accordingly, and if the transmission burst is small (i.e. it requires only a short transmission time) the packet transfer will already be over by then. Thus for small transmission bursts the CSI estimation and feedback drains the UE battery and the CSI reference symbols consume energy in the eNB, but these efforts provide limited performance gain.
In the closed-loop spatial multiplexing scheme defined for LTE Rel-8 and specified in the standard specification 3GPP TS 36.212, “Multiplexing and channel coding”, v. 8.8.0, a TPMI (transmit pre-coder matrix indicator) is sent in the downlink to the UE. With TPMI signaling it is possible to indicate that:                The pre-coding used is the same as what the UE reported in the last PMI report.        Alternatively an explicit PMI is indicated that is used on all scheduled sub-bands.        Furthermore, for robustness reasons it is possible to indicate that the current transmission uses transmit diversity (TX diversity). TX diversity is a robust transmission scheme, but it is single-rank, i.e. not multi-stream.        
There are also other challenges related to closed-loop spatial multiplexing than the issues mentioned above. Assume for instance that a certain UE has reported a rank indicator (RI) of e.g. 2 to an eNB, but the eNB has not yet received any PMI report from the UE. Which line of action is then the best for the eNB? The eNB may either select rank 2 with a fixed pre-coding matrix (but which one?), wait until a CSI report has been received (causing packet delay), or use TX diversity (robustness at the cost of rank loss). Alternatively the eNB can constantly request detailed CSI reports from UEs in case they might be needed, but such operation drains the UE battery.
There are thus a number of problems and challenges associated with prior art closed loop transmission schemes.